Although some edible oils are used per se, by far the largest portion are hydrogenated, or hardened, prior to their end use. The purpose of such hydrogenation is to increase the stability of the final product. For example, processed soybean oil is susceptible to oxidation resulting in deterioration of its organoleptic properties upon storage even at ambient temperature. Where the oil is to be used at higher temperatures, for example, as a frying oil, the adverse organoleptic consequence of oxidation become even more pronounced.
The commonly accepted origin of oxidative deterioration is attributed to highly unsaturated components, such as the triene moiety, linolenate, in soybean oil. Partial hydrogenation to remove most of this component leads to a marked increase in the oxidative stability of the resulting product, thereby facilitating storage and permitting unobjectionable use at higher temperatures. Ideally, one desires this hydrogenation to be highly specific, reducing only triene to the diene, linoleate, without affecting other unsaturated components and without effecting cis to trans isomerization. In practice, this goal is unachievable.
The edible fats and oils which are the subject of this invention, collectively referred to as fatty materials, are triglycerides of fatty acids, some of which are saturated and some of which are unsaturated. In vegetable oils, the major saturated fatty acids are lauric (12:0), myristic (14:0), palmitic (16:0), stearic (18.0), arachidic (20:0), and behenic (22:0) acids. The notation, "18:0," for example, means an unbranched fatty acid containing 18 carbon atoms and 0 double bonds. The major unsaturated fatty acids of vegetable oils may be classified as monounsaturated, chief of which are oleic (18:1) and erucic (22:1) acids, and polyunsaturated, chief of which are the diene, linoleic acid (18:2), and the triene, linolenic acid (18:3). Unhardened vegetable fats and oils contain virtually exclusively cis-unsaturated acids.
In the context of partial hydrogenation, the ultimate goal is the reduction of triene to diene without attendant trans-acid formation or saturate formation. In practice, it is observed that partial reduction results in lowering both triene and diene and increasing the monoene, saturate, and trans levels. Because it is desired that the product of partial hydrogenation itself be a liquid oil relatively free of sediment or even cloudiness upon storage at, for example, 10.degree. C., the formation of saturated and trans acids in such hydrogenation is a vexing problem. Removal of these solids, whose relative amount is measured by the Solid Fat Index (SFI), is a relatively costly and inefficient process attended by large losses associated with the separation of gelatinous solids from a viscous liquid. It is known in the art that such solids are composed largely of triglycerides containing at least one saturated fatty acid moiety and/or trans monounsaturated fatty acid moiety with the predominant culprits having at least 18 carbon atoms. It is further known in the art that fatty acid analysis alone may be an insensitive analytical tool, that is to say, two products of hydrogenation of, for example, soybean oil may be vastly different in their SFI while having virtually identical fatty acid analysis. This arises because the distribution of the saturated moieties in the triglyceride is important. The solubility in the soybean oil of disaturated triglycerides is much less than twice the amount of monosaturated triglycerides, and the solubility of monosaturated triglycerides may depend upon whether the other fatty acid moieties of the triglyceride are monounsaturated, diunsaturated, etc., and may also depend upon whether the saturated portion is at the one- or two- position of the triglyceride. Hence, hydrogenation of edible fats and oils is largely an empirical process, whose analytical tools most often include SFI supported by fatty acid analysis.
Group VIII metals on porous supports such as alumina, silica, and various clays as bentonite, long have been used as a catalyst in the hydrogenation of fatty materials. Partly because of its more favorable selectivity characteristics relative to other members, nickel appears to be the most widely used of such metals. But even nickel is only partially successful in achieving the ideal goal as elaborated above, one consequence of which is that for reductions of fatty materials to be sufficiently selective to meet commercial demands it is virtually obligatory to conduct the hydrogenation in a batch process.
We have discovered that when Group VIII metals are dispersed on porous supports with an eggshell distribution of the metal, the resulting catalytic materials give substantially improved selectivity in the hydrogenation of fatty materials. This observation forms the basis for the invention described herein, which is a method for selectively hydrogenating fatty materials by either batch or continuous process. The utility and importance of this invention is readily discerned when it is appreciated that commercial methods of continuous reductions of fatty materials are at once highly desirable and extraordinarily difficult, with no general method presently available for widespread industrial usage.
Metals dispersed on supports with an eggshell distribution are known. In U.S. Pat. No. 4,113,658 Geus describes a process of achieving such a distribution and requires quite small particles of the carrier as a nucleating agent for the deposited metal. The patentee ascribes increased catalytic activity to such preparations, resulting from a higher surface area. U.S. Pat. No. 4,128,506 describes a process wherein a layered catalyst, effective in automotive emission control, may be produced. U.S. Pat. No. 4,376,724 teaches a rhodium dispersed on silica or titania with an eggshell distribution where the purpose of such a distribution is to achieve economy in metal utilization, the patentee appreciating that metals in the interior of a support often do not participate, or participate to a lesser extent, in catalytic reactions. However, in none of these do the patentees recognize that deposition of a metal on a porous support with an eggshell distribution may have an appreciable effect on selectivity in catalytic hydrogenation of polyunsaturated compounds as are found in fatty materials.